1. Field of the Invention
The invention relates to the area of combustion technology. It relates to a pressure atomizer nozzle comprising a nozzle body in which there is formed a turbulence and/or swirl chamber which is connected via a nozzle bore to an outer space and has at least one feed channel for the liquid to be atomized, through which said liquid can be fed in under pressure, and to a method for operating this pressure atomizer nozzle.
2. Discussion of Background
Atomizer burners in which the oil which is burnt is finely divided by mechanical means are known. The oil is broken up into fine droplets of about 10 to 400 .mu.m diameter (oil mist) which vaporize and burn when mixed with the combustion air in the flame. In pressure atomizers (see Lueger--Lexikon der Technik, Encyclopedia of Technology!, Deutsche Verlags-Anstalt Stuttgart, 1965, Volume 7, p 600), the oil is fed to an atomizer nozzle under high pressure by an oil pump. Through essentially tangentially extending slots, the oil passes into a swirl chamber and leaves the nozzle via a nozzle bore. This ensures that two components of motion, an axial component and a radial component, are imparted to the oil droplets. The oil film emerges from the nozzle bore as a rotating hollow cylinder and expands due to the centrifugal force to form a hollow cone whose edges enter into unstable vibration and break up into small oil droplets. The atomized oil forms a cone with an aperture angle of greater or lesser size.
However, in the case of combustion of mineral fuels with low pollutant emissions in modern burners, for example in premix burners of the double-cone type, the basic structure of which is described in EP 0 321 809 B1, special requirements are made of the atomization of the liquid fuel. These are, in particular, the following:
1. The droplet size must be small to ensure that the oil droplets can vaporize completely before combustion. PA0 2. The opening angle (angle of spread) of the oil mist should be small, especially in the case of combustion at elevated pressure. PA0 3. The droplets must have a high speed and a high momentum in order to be able to penetrate sufficiently far into the compressed mass flow of combustion air and thereby ensure that the fuel vapor can premix completely with the combustion air before it reaches the flame front.
Swirl nozzles (pressure atomizers) and air-assisted atomizers of the known types with a pressure of up to about 100 bar are hardly suitable for this purpose because they do not permit a small angle of spread, the quality of atomization is limited and the momentum of the droplet sprays is low.
As a result of this inadequate vaporization and premixing of the fuel, addition of water is therefore necessary for local reduction of the flame temperature and hence of NO.sub.x formation. Since the water fed in often also disturbs flame zones, which, although producing little NO.sub.x themselves, are very important for flame stability, instabilities, such as flame pulsation and/or poor burn-up, often occur, leading to an increase in CO emissions.
An improvement can be achieved with the high-pressure atomizer nozzle disclosed in EP 0 496 016 B1. This comprises a nozzle body in which there is formed a turbulence chamber which is connected via at least one nozzle bore to an outer space and has at least one feed channel for the liquid to be atomized, which can be fed in under pressure. It is distinguished by the fact that the cross sectional area of the feed channel opening into the turbulence chamber is greater by a factor of 2 to 10 than the cross sectional area of the nozzle bore. This arrangement makes it possible to produce a high level of turbulence in the turbulence chamber and this does not die away on the way to the outlet from the nozzle. The jet of liquid is induced to break up rapidly by the turbulence produced in the outer space in front of the nozzle bore, i.e. after it leaves the nozzle bore, small angles of spread of 20.degree. and less being obtained. The droplet size is likewise very small.
When operating gas turbine burners with liquid fuel, the aim is to produce, as far as possible over the entire load range of the gas turbine (about 10% to 120% of fuel mass flow in relation to rated load conditions), a droplet spray which allows stable combustion with low pollutant emissions in a predetermined airflow field in the entire range.
It is true that, as desired, the use of a high-pressure atomizer nozzle as described above for atomizing liquid fuel in gas turbine burners leads to a not excessively high pressure (100 bar) and a small droplet size at full load and overload (100-120%), unwanted wall wetting and carbonization being avoided by virtue of the narrow spray angle.
At partial load, however, the fuel feed pressure falls due to the falling total fuel mass flow. The energy required for atomization for pressure atomizers is, however, determined by the fuel feed pressure, with the result that there is a deterioration in the quality of atomization in this load range and the depth of penetration of the fuel spray into the airstream is decreased due to the low fuel feed pressure.